1. Field of the Invention
The present invention relates to a limit circuit used in an FM receiver and so on.
2. Description of the Related Art
An amplitude of an FM wave received by an FM receiver is not constant because it is influenced by fading, a multipath and so on. If such a received wave is FM-detected as it is, distortion and noise are mixed in detection output, and so a limit circuit is generally used. In particular, a limit amplifier of a high gain is used as the limit circuit recently.
Incidentally, in order to realize the limit amplifier of a high gain as mentioned above, it is necessary to connect amplifiers of a relatively low gain in multiple stages. For instance, in the case of realizing a gain of 90 dB or so and using a differential amplifier of 13 dB or so in each stage, it is necessary to set the number of stages at seven stages.
However, there is a problem that in the case of the high gain of 90 dB or so, the amplifiers are saturated with noise components included in a signal which is a subject of amplification, so that the high gain of 90 dB or so which is realizable in design cannot be obtained in reality. For instance, it was verified that, when operation of the above-mentioned limit circuit comprised of seven-stage differential amplifiers was simulated by using a simulation tool and an experiment was performed by actually building the circuit, only the gain of 60 dB or so could be obtained.